VETERINARY
Does bleeding affect performance? A ten-yeAr study
Capillary stress failure
When looking for blood within the trachea (windpipe), it is important that sufficient time is given for the blood to have travelled up from the deepest parts of the lungs. A minimum of 30 minutes is considered ideal
Exercise-induced pulmonary hemorrhage, also known as EIPH or “bleeding,” is a common problem in racehorses. Many trainers know from bitter experience that it can have a significant impact on some horses’ performance. Yet, other horses that bleed continue to race effectively. A recent issue of Equine Veterinary Journal (EVJ) contained three exciting new studies, each of which looked at the impact of bleeding on different aspects of race performance. These studies were accompanied by an article by Professor N. Ed Robinson, a leading EIPH researcher from the University of Michigan, which summarized what we know about how bleeding arises and explained why some horses are affected more than others. WORDS: PROFESSOR CElia M. MaRR, EquinE VEtERinaRy JOuRnal PHOtOS: lEWiS SMitH, ROSSDalES EquinE HOSPital anD DiagnOStiC CEntRE
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The racehorse is fueled by oxygen, and an extremely efficient heart is needed to carry this fuel from the lungs to the body. At peak output, the equine heart is pumping up to 450 liters of blood each minute and beating around four times every second. To operate at this rate, the heart must work at a very high pressure that is also maintained in the blood vessels. In the lung, the transfer of oxygen occurs within the capillaries. These form a meshwork of very fine vessels around the air sacs: the thinner the capillaries, the more efficient the transfer of oxygen. But the thinner the capillaries, the less they can resist the pressure needed to fill the heart. EIPH happens when the pressure inside the capillaries causes them to burst, with leakage of blood into the airways. In a small proportion of horses that bleed, the amount of blood is sufficient to appear at the nostrils. Capillaries in the upper part of the lung, immediately in front of the diaphragm, tend to be most affected and this is generally the site affected by EIPH. The capillary stress failure theory was first put forward in the mid 1990s and it explains why EIPH occurs but it does not shed any light on why some horses bleed more than others or why bleeding tends to get worse over time. Professor Robinson and his colleagues have spent several years unraveling the much more complex events that are at play.
Venous remodeling
Robinson’s team has shown with detailed pathology studies that horses that bleed have scar tissue within the walls of the small veins in the lung. As the walls thicken, the veins become less stretchy, causing a backing up of pressure into the connected capillaries, making them prone to burst. Robinson suggests that venous remodeling